1. Field of the Invention
The present invention pertains to the field of personal computer systems. More particularly, this invention relates to a personal computer system telephone line sharing among voice, data modem, and fax calls.
2. Art Background
Non dedicated telephone lines are often employed in small office or home office environments to provide voice, modem, and fax communication for a personal computer, as well as for other telephone devices that are present. The personal computer typically employs one non dedicated telephone line to receive modem and fax calls as well as voice calls. Such telephone line sharing enables modem and fax communication while avoiding the cost of providing separate modem telephone lines and fax telephone lines in the small business or home office.
In such small business or home office environments, a telephony subsystem of the personal computer typically performs the telephone line interface functions required for modem, fax and sometimes voice communication. The telephone line interface functions include line seizure and ring detect, as well as audio and digital signal processing functions.
Typically, application programs executing on the personal computer perform the higher level modem, fax and voice communication functions as well as user interface functions. For example, a fax application program on the personal computer controls fax transfers over the shared telephone line through a driver program for the telephony subsystem.
In a typical small business or home office telephone line topology, the telephony subsystem is coupled to an incoming telephone line or a private branch exchange (PBX) line. One or more extension telephones and a telephone answering machine are typically coupled to the incoming telephone line in parallel with the telephony subsystem. The telephony subsystem usually drives one or more serial telephones over a local line.
Typically in such systems, the telephony subsystem initially determines whether an incoming call is a fax by going off hook and thus picking up the line, and attempting to sense a fax calling tone while transmitting a simulated ringback tone over the telephone line. If the fax calling tone is detected, then the telephony subsystem picks up the telephone line and transmits a fax response tone over the telephone line. Thereafter, the fax application program performs a fax data transfer through the telephony subsystem.
If the telephony subsystem does not detect the fax calling tone with the incoming call, then the incoming call is typically assumed to be a voice call. The telephony subsystem rings the serial telephone to enable a user to receive the voice call and couples the incoming line to the local line when the serial telephone is picked up.
Unfortunately, such prior telephony subsystems cannot properly process incoming calls from a data modem and manual fax. Such devices call and wait for a response tone rather than transmit a calling tone. As a consequence, the telephony subsystem in such a prior system usually assumes that an incoming call from a data modem or manual fax is a voice call and rings the serial telephone. The user picking up the serial telephone perceives silence on the telephone line because the data modem or the manual fax is waiting for a response tone.
Moreover, such telephony subsystems ring only the serial telephones coupled to the local line. As a consequence, the parallel telephones coupled to the incoming telephone line do not achieve full utility.